1. Field of the Invention
This invention relates generally to a propeller used to produce thrust when driven by a marine propulsion system, and more particularly, to a marine propeller which is capable of replacing a conventional marine propeller and includes a series of arcuate ribs on each blade used to increase hydrodynamic efficiency.
2. Description of the Prior Art
A propeller often is defined as a bladed device that rotates on a shaft to produce thrust in the direction of the shaft axis. Propellers are used in a wide variety of applications, including mechanical and electrical air fans, aircraft, and boating. In a marine environment, the propeller attached to a motor is the means by which the vessel or boat is driven through the water. Marine propellers may operate either clockwise or counterclockwise in relation to the boat's stern and may be shaft or stern-driven. Also, marine propellers are manufactured in a variety of shapes and sizes, depending on application. These types include standard outboard, cleavers, modified cleavers, choppers, and "elephant ears." As the propeller is driven or rotated by the motor, a negative pressure is created on one side of each blade surface while a positive pressure is created on the blade's opposite side. This pressure differential causes water to be drawn into the submerged propeller from an area forward of the rotating blade where it is then pushed or accelerated at a high velocity from its rear.
In the past a number of methods have been used to modify the marine propeller in order to increase propeller efficiency. Increased efficiency allows the propeller to produce more thrust per revolution. One commonly used method is where two or more of the blades of the propeller are offset in a forward direction from a perpendicular line extending through the center of the blade hub. This offset is called the blade rake and increases a boat's ability to operate in both cavitational and ventilation situations. Propeller cavitation is generally the formation of both combustion exhaust vapor and air filled bubbles or cavities in water near or on the surface of the rotating propeller. These cavities occur when their pressure falls below the vapor pressure of water. Ventilation occurs when the blade is either fully or partially exposed from the water while in operation.
A second and more common modification to the propeller includes blade pitch adjustment which is the amount of twist or turn of each blade in relation to the propeller hub. This twist allows the blade to form a helical surface. The propeller pitch is commonly set to determine the amount of water which will be pulled through the propeller per revolution, which in turn produces a corresponding amount of thrust.
A third and less often used method to increase propeller efficiency has been the addition of a supplementary vane or rib blade which extends perpendicular to the propeller surface. This rib helps to draw water from the outer periphery of the blade towards the blade root or hub. This has the overall effect of increasing propeller efficiency since the ratio of thrust horsepower produced by the propeller is increased in relation to the shaft horsepower as delivered by the motor to the propeller. British patent 9930 discloses the use of a propeller which utilizes additional rib blades positioned on a main blade. The ribs protrude from the blade surface at a uniform height and extend linearly from leading edge to trailing edge.
Additionally, U.S. Pat. Nos. 4,757,587, 4,128,363, and 3,294,175 further teach the use of rib configurations which extend across the blade surfaces in order to enhance propeller blade effectiveness. Although these ribs may be present on the blade surface, each rib does not include a cup area, nor are the ribs staggered in height from the center of the hub to the outer tip, nor are they tapered from a leading blade edge to trailing blade edge.